As for the convictions you are speaking of, those were in a Jordanian court, If i am not mistaken, and there is some question about them.
Furthermore, Chalabi was not the source of the information, he put those with the information in touch with intelligence agencies. Some of the information provided by the Iraqi National Congress has proved very good, some has been so-so, and some has been either worthless or yet to be borne out.
Using your reasoning, we should turn away offered intel because chalabi was convicted of a crime in an arab court (the epitomy of justice applied), and remain completely ignorant, rather than try and make use of what information is available.
Interestingly enough, a lot of what the Iraq National Congress said jived with what the U.N. and other sources had.
All of the stories of atrocities that Chalabi's group told have been proven in spades. Evidence has been found to support much of what they provided concerning WMD's (mobile labs have been found, as well as several "dual-use" facilities capapble of producing WMD's, and just a few days ago several soviet missles with dirty bombs were found).
Hiding and destroying evidence of chemical and bio weapons is quite easy, and Saddam had plenty of time to do it.